5 Worlds Where Extraterrestrial Life Could Be Hiding

For centuries, humanity has looked to the stars and planets and wondered if life could have taken root elsewhere in the universe, beyond the fragile marble we call home. Philosophers first began recording their thoughts on the subject over 2,000 years ago, with the Roman poet Lucretius positing that there must be other worlds in the universe populated with life.

Although Lucretius' ideas were very much on the fringe for the next 1,500 years, by the time of the Renaissance, the idea of life on other planets was beginning to be reconsidered. Upon the discovery of moons around Jupiter in the 1600s, German astronomer Johannes Kepler reasoned that it must have life just as Earth does.

Today the search for extraterrestrial life is more formalized and primarily undertaken by national space agencies, as well as private organizations like the SETI Institute and Breakthrough Listen. The search for life can take many forms: NASA has the privilege of being able to directly analyze rocks on Mars, whereas the SETI Institute relies on listening for radio signals. So far, no one has been able to definitively show that life exists on other worlds, but some tantalizing clues have been found.

Mars may be a cold and unforgiving place now, but that wasn't always the case. In fact, many researchers think that Mars used to have an ocean and an atmosphere thick enough to prevent it from evaporating. In other words, a perfect place for life to flourish.

But that was billions of years ago and the Mars of today is a very different place. It's cold, it's dry, and there's virtually no atmosphere; not the kind of place you'd want to vacation. Still, Mars has lots of water — a requirement for life as we know it — trapped underground or beneath ice caps, protected from the lethal solar radiation, and it's here that scientists think life could be hiding.

If life is found on Mars, it's theorized that it will be rudimentary, although simulations have shown that single-celled organisms can survive in an environment similar to Mars. The red planet also has more methane than it should, which could be a sign of life or just an odd coincidence of geology. It's also possible that we've already seen Martian life. Some scientists (albeit a fringe group) believe that the Opportunity rover inadvertently photographed fungus and lichens on Mars, but other scientists are skeptical.

In order for life as we know it to develop, it needs a few things. First, it needs the right blend of chemicals to build the vital molecules. Next, it needs some form of energy to power the chemical reactions. Finally, it needs a medium to slosh around in to make interesting chemical reactions more likely to occur. Earth has all of these elements, but it's not the only big rock floating around the sun that does.

It's long been hypothesized that Jupiter's moon Europa has liquid water. Its remarkably smooth surface is nearly devoid of craters, indicating a young surface, and the pattern of cracks that mar its face hint that the surface crust floats over a liquid interior. Mysterious plumes of water vapor have also been detected erupting from the surface, which many think is a sure sign of a subsurface ocean.

As for the other two essential ingredients, scientists can only speculate, but the evidence is promising. Life's basic building blocks are common in the universe so it would be odd if they weren't present on Europa, too. And those big cracks on Europa are evidence that the surface is bending and flexing under tidal pressure, pressures that could produce hot vents at the bottom of Europa's ocean; similar vents on Earth were discovered in the '70s and are rife with life.

Between the orbits of Mars and Jupiter, the asteroid belt isn't generally thought of as a potential location to search for signs of life, past or present, but recently, scientists have found some intriguing signs that life may be possible on the king of the asteroids, Ceres. Before 2015, our knowledge of Ceres was largely confined to what we could infer from its orbit, and what we could see from blurry images taken by Hubble. Following the arrival of the Dawn spacecraft to Ceres in early 2015, our knowledge has grown by leaps and bounds.

The first hint of water on Ceres occurred in 2014 when the Herschel Space Observatory used infrared spectroscopy to observe water vapor at more than one location, suggesting that the dwarf planet's mantle (the part between the crust and the core) is composed of water ice. Following the Dawn mission, it was found that Ceres does have an icy mantle and a few other potential biomarkers.

For one, organic compounds have been discovered on Ceres. If those chemicals were created in situ and not the product of an errant comet impact, the possibility for life goes up. Finally, even if Ceres isn't tidally active like Europa is, it could maintain reservoirs of liquid water by radioactive heating or by virtue of maintaining such a high salinity that the water cannot melt.

If one were to look at just the surface conditions of Venus, it wouldn't seem possible that life could exist there. Unlike the gentle 14.7 pounds per square inch of pressure one receives on Earth, Venus delivers a crushing 1,350 psi. Worse, one would have to contend with temperatures over 800°F, hot enough to melt lead. But, Venus is more than just the hellscape of its surface.

Starting at about 15 miles above the surface, the climate is much less oppressive. There, nestled among Venus' thick blanket of clouds, the temperature and pressure approach levels similar to those on Earth, and more importantly, it has all of the chemical ingredients needed for life. Certain types of life have been found living in Earth clouds, so the idea of life in Venusian clouds isn't so far-fetched.

No direct evidence for life on Venus has been found, but there are some clues that it may be lurking there. In 2021, a group of scientists announced the detection of phosphine gas in the Venusian atmosphere, concluding that there was no known mechanism for phosphine production on Venus that doesn't involve life. In fact, phosphine is considered a "biosignature gas" among astrobiologists. Still, even a whisper of extraterrestrial life sets space researchers on edge and many other potential sources for the gas have been put forward, but no single theory is conclusive.

Our star is only one among the hundreds of billions in the Milky Way, and among those stars similar to the sun, about half are estimated to have rocky planets orbiting in their habitable zones. That means there are a lot of planets out there that could harbor life, if only we could detect it.

One of the most promising candidates so far is HD 20794 d, which orbits a star less than 20 light years away. Based on the mass of HD 20794 d (around six times more massive than the Earth), it's possibly a terrestrial planet, and despite its elliptical orbit, it spends most of its time within its star's habitable zone. The researchers speculate that life could find a way there if it has a significant ocean and the proper atmosphere.

And HD 20794 d is far from the only candidate for life outside of our solar system. In 2023, the James Webb Space Telescope managed to detect methane and carbon dioxide in the atmosphere of K2-18 b, which also orbits in its star's habitable zone. Scientists are particularly excited about K2-18 b because they might have found evidence of dimethyl sulphide, a gas which, on Earth, is only produced by life.